(1) Field of the Invention
The present invention relates to the determination of cardiac output of a patient by injecting a measured amount of cold injectate into the right heart proximal to the pulmonary artery in a predetermined time of short duration. More specifically, this invention is directed to a thermodilution injector including a syringe and an associated injectate cooling device and, more particularly, an injectate cooler for use in a thermodilution injector. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
A well known test for the determination of cardiac output involves the injection of a measured amount of cold injectate solution into the right heart proximal to the pulmonary artery in a predetermined time period of short duration, such as, on the order of two seconds. The temperature drop of the blood passing a thermistor positioned in the heart is then sensed and measured. The decrease in blood temperature in a given time resulting from the cold injectate solution, when integrated by a cardiac output computer, is a measure of the output capacity of the heart in liters per minute. This technique for determining cardiac output is well known and is of considerable importance in diagnosing and treating critically ill patients. The value of the technique of thermodilution cardiac output monitoring is directly related to the accuracy of the process. Many thermodilution cardiac output computers are commercially available for obtaining determinations of cardiac output from a blood temperature drop curve.
The reliability of the technique of thermodilution cardiac output monitoring depends on the accuracy and the repeatability of the injection process. It has been found to be difficult to provide a predetermined amount of injectate having a predetermined cold temperature on a repeatable basis. In order for the output readings to be accurate, repeatable and reliable, the injectate must have a predefined constant temperature and must be delivered to the patient over a short predetermined period of time, which must be the same for each injection. If the temperature of the injectate varies, the rate of change of blood temperature over a given time will also vary, and the computer output readings will thus be inaccurate and unreliable. Bearing in mind that injection should occur over a time period of approximately two seconds, the time it takes for 10 cc of dextrose solution to be injected manually, it can readily be seen that a variation of only two degrees centigrade in temperature of the injectate can lead to substantial errors in measurement.
In the present method of thermodilution injection, a doctor or medical technician operates a syringe to deliver the injectate into a catheter placed in the right heart proximal to the pulmonary artery. The injectate is typically stored in a bottle which is positioned in an ice bath. Injectate having a temperature of 0.degree. C. is delivered to the syringe. During transference of the injectate to the syringe and during the time in which the injectate remains in the syringe, the temperature of the injectate may increase. Moreover, the apparatus for cooling the injectate is cumbersome and difficult to operate. Since the cooled injectate must be transferred to the syringe, there exists a possibility of contamination of the injectate during transfer.